The functional mechanism behind the latitudinal pattern of liana diversity: Freeze-thaw embolism reduces the ecological performance of liana species.

There is a strong decrease in liana diversity along latitudinal and altitudinal gradients at a global scale, and there is a marked difference in liana diversity between tropical and temperate ecosystems. From these observations, it has been proposed that cold temperatures would restrict the ecological patterns of liana because of their vascular system's vulnerability to freeze-thaw embolism. Our objective was to establish the functional mechanism that drives the loss of liana diversity along a latitudinal temperature gradient. We evaluate the ecological performance of liana in 10 different species based on the apical growth rate, as well as functional traits associated with efficiency (maximum hydraulic conductivity and percentage conductivity lost) and safety of water transport (vessel diameter, vessel density, wood density, and root pressure). We found that at the colder (more southern) site within the latitudinal gradient, liana species showed lower performance, with a fivefold decrease in their apical growth rate as compared to the warmer (more northern) sites. We postulate that this lower performance results from a much lower water transport efficiency (26.1-fold decrease as compared to liana species that inhabit warmer sites) that results from higher freeze-thaw (37.5% of PLC) and reduction of vessel diameter (3 times narrower). These results are unmistakable evidence that cold temperature restricts liana performance: in a cold environment, liana species exhibit a strong decrease in performance, low efficiency, and higher safety of water transport. Conversely, at warmer sites, we found that liana species exhibit functional strategies associated with higher performance, higher efficiency, and lower safety of water transport capacity. This trade-off between efficiency and safety of water transport and their effects on performance could explain the latitudinal pattern of liana diversity.

Hydraulic differences along the water transport system of South American Nothofagus species: do leaves protect the stem functionality?

Hydraulic traits were studied for six Nothofagus species from South America (Argentina and Chile), and for three of these species two populations were studied. The main goal was to determine if properties of the water conductive pathway in stems and leaves are functionally coordinated and to assess if leaves are more vulnerable to cavitation than stems, consistent with the theory of hydraulic segmentation along the vascular system of trees in ecosystems subject to seasonal drought. Vulnerability to cavitation, hydraulic conductivity of stems and leaves, leaf water potential, wood density and leaf water relations were examined. Large variations in vulnerability to cavitation of stems and leaves were observed across populations and species, but leaves were consistently more vulnerable than stems. Water potential at 50% loss of maximum hydraulic efficiency (P(50)) ranged from -0.94 to -2.44 MPa in leaves and from -2.6 to -5.3 MPa in stems across species and populations. Populations in the driest sites had sapwood and leaves more vulnerable to cavitation than those grown in the wettest sites. Stronger diurnal down-regulation in leaf hydraulic conductance compared with stem hydraulic conductivity apparently has the function to slow down potential water loss in stems and protect stem hydraulics from cavitation. Species-specific differences in wood density and leaf hydraulic conductance (K(Leaf)) were observed. Both traits were functionally related: species with higher wood density had lower K(Leaf). Other stem and leaf hydraulic traits were functionally coordinated, resulting in Nothofagus species with an efficient delivery of water to the leaves. The integrity of the more expensive woody portion of the water transport pathway can thus be maintained at the expense of the replaceable portion (leaves) of the stem-leaf continuum under prolonged drought. Compensatory adjustments between hydraulic traits may help to decrease the rate of embolism formation in the trees more vulnerable to cavitation.

Ecophysiological traits may explain the abundance of climbing plant species across the light gradient in a temperate rainforest.

Climbing plants are a key component of rainforests, but mechanistic approaches to their distribution and abundance are scarce. In a southern temperate rainforest, we addressed whether the dominance of climbing plants across light environments is associated with the expression of ecophysiological traits. In mature forest and canopy gaps, we measured leaf size, specific leaf area, photosynthetic rate, and dark respiration in six of the most abundant woody vines. Mean values of traits and their phenotypic change (%) between mature forest and canopy gaps were predictor variables. Leaf size and specific leaf area were not significantly associated with climbing plant dominance. Variation in gas-exchange traits between mature forest and canopy gaps explained, at least partly, the dominance of climbers in this forest. A greater increase in photosynthetic rate and a lower increase in dark respiration rate when canopy openings occur were related to the success of climbing plant species. Dominant climbers showed a strategy of maximizing exploitation of resource availability but minimizing metabolic costs. Results may reflect phenotypic plasticity or genetic differentiation in ecophysiological traits between light environments. It is suggested that the dominant climbers in this temperate rainforest would be able to cope with forest clearings due to human activities.